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Issue 49: Fire Safety for Hydrogen Fuel Cell Vehicles

By Carl H. Rivkin, P.E.

Hydrogen fuel cell vehicles have been deployed at the prototype level
in the US and other countries such as Japan and Germany. However,
planned commercial deployment will occur in the near future, and it is
therefore important to have a basic understanding of the safety issues
that these vehicles present.

Hydrogen fuel cell vehicles present different hazards than gasoline
powered vehicles do. These hazards require taking different safety
measures than those employed on gasoline-powered vehicles. As with
gasoline powered vehicles, these hazards can be addressed with both
engineering and operator controls.

There are several ways of classifying hazards associated with
hydrogen fuel cell vehicles. This article will group hazards into three
areas: dispensing and operating, onboard hydrogen storage and use, and
onboard electrical systems. This article does not address vehicles that
employ hydrogen internal combustion engines, although these vehicles
present many of the same hazards as hydrogen fuel cell vehicles.

A hydrogen fuel cell vehicle consists of the systems shown in Figure
1. A hydrogen fuel cell vehicle is an electric vehicle because it is
powered by an electric motor. The fuel cell stack converts hydrogen into
electricity that powers the motor. The term "stack" is used because the
conversion device is composed of many fuel cells that operate in series
to produce the electric current required to run the motor. A single
fuel cell would not produce sufficient power to meet the required
performance parameters.

Vehicles typically use Polymer Electrolyte Membrane (PEM) fuel cells,
although there are other fuel cell technologies. Hydrogen is fed on the
anode side of the fuel cell and oxygen is fed on the cathode side of
the fuel cell. The PEM allows the hydrogen molecule that has separated
into OH - and H + ions to have the OH - ion flow through the membrane
and the H + ion to flow around the membrane in a circuit. PEM fuel cells
require less time to power-up and operate at lower temperatures than
other fuel cell technologies, which makes them suitable for vehicle
applications.

Figure 1. Hydrogen Fuel Cell Vehicle Schematic (Source NREL)

Dispensing Operations

Dispensing gaseous hydrogen into fuel cell vehicles presents hazards
that are different than dispensing gasoline; hydrogen is transferred
into the vehicle at pressures that may exceed 10,000 psi (70 MPa). The
dispensing nozzle must connect to the vehicle so that, unlike gasoline
dispensing, it forms a closed, high-pressure system. Failures of the
nozzle to lock to the vehicle should terminate the dispensing operation.
However, there is still an opportunity to release hydrogen in the event
of a nozzle failure or other leak.

The failure of the nozzle to adhere properly to the vehicle also
presents the hazard of an unrestrained high-pressure hose proximate to
the individual performing the fueling. This hazard is addressed
primarily by engineering the nozzle system for very high reliability.

Hydrogen storage and use on-board the vehicle

Almost all hydrogen fuel cell vehicles use high-pressure gaseous
storage systems to provide the hydrogen for the fuel cell stack. There
are cryogenic /compressed hydrogen storage systems that are being
developed, but the majority of vehicles that will be deployed relatively
soon will use high pressure storage systems. These systems are
typically either 5000 psi (35 MPa) or 10000 psi (70 MPa) systems.

Hydrogen storage systems are designed with sensor technologies that
will shut down the system if a leak is detected. The leaks are typically
detected by sensing a pressure drop in the system. If the system is
exposed to fire, the pressure relief device (PRD) will function and
relieve the contents of the tank. SAE J25781 states that the
PRD should be installed to direct flow away from people, assuming the
orientation of the vehicle is not changed from the routine orientation.

There is not standardized labeling system used on hydrogen fuel cell
vehicles, although most vehicles have some type of label that indicates
that hydrogen is on the vehicle.

The storage systems will also have to meet the Federal Motor Vehicle
Safety Standards (FMVSS) for crash safety so that the systems will
maintain their integrity after the specified crash conditions.

Hydrogen fuel cell vehicles use an electric motor to power the
vehicle. This motor requires high voltage lines to run from the fuel
cell, typically located in the center of the vehicle, to the electric
motor located in the front of the vehicle. Hydrogen fuel cell vehicles
would also typically employ a capacitor or battery to allow storage to
start the vehicle and compensate for variation in fuel cell power
output. There are a few basic safety rules that emergency responders
should follow:

Never cut into hydrogen lines (No standard markings, most are silver (stainless steel))

About SFPE

SFPE is a global organization representing those practicing in the fields of fire protection engineering and fire safety engineering. SFPE’s mission is to define, develop, and advance the use of engineering best practices; expand the scientific and technical knowledge base; and educate the global fire safety community, in order to reduce fire risk. SFPE members include fire protection engineers, fire safety engineers, fire engineers, and allied professionals, all of whom are working towards the common goal of engineering a fire safe world.